The present invention relates to the field of leakproof paper cups and the making thereof.
Volume production of paper cups and plastic coated paper cups has achieved considerable success in the United States of America and abroad. One deficiency inherent in the cups formed using a layer of insulating foam is that the leak rate per million cups is almost double when compared to cups formed using plain paper stock. However, insulating foam cups have proven to be very popular. An unsolved problem is how to produce a better bottom seam seal for insulating foam cups since most of the leaks occur where the side seam adjoining a lower cylindrical portion of the cup attaches to the bottom blank.
FIG. 1A shows a cross section of a prior art cylindrical cup 100. The peripheral wall 150 of cup 100 has its lower edge 160 curved inwardly and upwardly forming a channel.
The circular bottom blank 110 is typically a single layer of board and has a downwardly extending skirt 120 formed about its periphery.
FIG. 1A also shows where the prior art applies heat to seal the bottom blank 110 to the peripheral wall 150. Heat is applied at 130 to the interior of the shallow hollow formed below the ascending sidewall 140 and the bottom blank 110.
FIG. 1B shows a more detailed cutaway of an X-point 180 where five layers of board come together. The X-point 180 is especially prone to leaks.
Typically, a single layer of board is shaped to form a peripheral wall 150 of a cup 100. The two edges of the peripheral wall board meet and overlap to form a side seam 170. The side seam 170 is created by overlapping the two edges of the board and sealing them together. Because the board is overlapped to form the side seam 170, the side seam 170 is two layers thick. Because the peripheral wall 150 has its lower edge 160 curved inwardly and upwardly to form the channel, a double layer of the peripheral wall 150 is thus also curved inwardly and upwardly at the seam 170.
At all points where the peripheral wall 150 meets the bottom blank 110, except at the X-point 180, there are three layers of board. To seal the peripheral wall 150 to the bottom blank 110, the heat must penetrate two layers of board. The two layers of board are the peripheral wall""s 150 single layer and the bottom blank""s 110 single layer.
But, at the X-point 180 is where side seam 170 meets the periphery of bottom blank 110, the side seam 170 is four layers thick, two layers on the outside and two layers on the inside where the side seam 170 is folded upward. When the bottom blank 110 is then coupled to the top blank, the X-point 180 becomes five layers thick. To seal at the X-point 180, the heat must penetrate three layers of board. The three layers of board are the side seam""s 170 two outer layers and the bottom blank""s 110 single layer.
The present invention provides a cup less likely to leak by forming a u-shaped channel about a periphery of a bottom blank, forming a top blank into a generally cylindrical shape, and coupling an edge of the top blank to the u-shaped channel of the bottom blank.